Early Transition Metal and f-Metal Chemistry

The research group's initial work with early transition metals focused on d0 metal-silicon bonds and their potential to undergo fundamentally new transformations such as CO insertion.1,2 A significant aspect of this research characterized "σ-bond metathesis" reactions as forming the basis for a new polymerization mechanism, by which early metal complexes catalyze the dehydropolymerization of hydrosilanes to polysilanes.3 This chemistry was extended to the polymerization of secondary stannanes, to produce the first high molecular weight polystannanes.4,5

Recent Projects

This σ-bond metathesis chemistry is potentially useful for the design of catalytic cycles, since it involves bond-making and bond-breaking events, but it has yet to play a prominent role in the development of new C–H activation catalysis. In work toward this goal, we reported a non-degenerate σ-bond metathesis reaction of the neopentyl complex Cp*2ScCH2CMe3 with methane under mild conditions to give neopentane and Cp*2ScMe. An analogous transformation appears to be involved in the catalytic hydromethylation of propene to isobutene, with insertion of propene into a Sc-Me bond followed by σ-bond metathesis of the insertion product, Cp*2ScCH2CHMe2, with CH4.6 The observation of catalytic turnover for this hydromethylation is relevant to conversion of abundant methane directly into chemical feedstocks and transportation fuels. Subsequent investigations have been oriented toward the optimization of this catalysis, via identification of the factors that govern activity and selectivity. Along these lines, scandium complexes of the ansa ligand Me2Si(C5Me4)2 possess a more exposed and more electrophilic metal center, and this leads to more rapid reactions of methane with scandium alkyl derivatives.7 

A second type of productive methane activation was found in studies of σ-bond metathesis reactions of silanes with Cp*2ScMe, which give methane (and a Sc silyl complex) or methylsilanes (and Cp*2ScMe), through concerted, four-centered transition states. These observations suggested that C–H and Si–H bond activations might be coupled into a catalytic cycle for the dehydrogenative silylation of hydrocarbons, and this was demonstrated in the Cp*2ScH-catalyzed reaction of methane with Ph2SiH2 to give Ph2MeSiH, and silylations of cyclopropane (to Ph2(c-C3H5)SiH) and isobutene (to Ph2(Me2C=CH)SiH).8 Related σ-bond metathesis chemistry involving C–H activations in arenes, and Si–C bond formation, is possible in lanthanide complexes such as those based on the Cp*2Sm fragment.9

The search for new catalysts that operate via σ-bond metathesis includes pursuit of both metal- and ligand-centered design principles. For example, the cationic nature of the hafnium complex [Cp2Hf–SiHMes2]+ results in an Si–H α-agostic interaction and dramatic promotion of benzene C–H activation.10 Other efforts have focused on sterically demanding, strongly electron-donating ancillary ligands such as permethylfluorenyl.11 

Studies with sterically demanding di(phosphino)pyrrolide ligands have led to discovery of C–H activations with scandium, and in one case this reactivity leads to an unexpected isobutene elimination corresponding to the retro-(2+2) cycloaddition that is a key step in olefin metathesis.12

References
1. "Insertion of Carbon Monoxide into a Transition-Metal-Silicon Bond. X-ray Structure of the Silaacyl (η5-C5H5)2Zr(η2-COSiMe3)Cl." T. D. Tilley. DOI: 10.1021/ja00299a058
2. "Carbonylation Chemistry of the Tantalum Silyl (η5-C5Me5)Cl3TaSiMe3. Synthesis, Characterization, and Reaction Chemistry of (η5-C5Me5)Cl3Ta(η5-COSiMe3) and Derivatives." J. Arnold, T. D. Tilley, A. L. Rheingold, S. J. Geib and A. M. Arif. J. Am. Chem. Soc. 1989, 111, 149-164. DOI: 10.1021/ja00183a026
3. "The Coordination Polymerization of Silanes to Polysilanes by a "σ-Bond Metathesis" Mechanism. Implications for Linear Chain Growth." T. D. Tilley. Acc. Chem. Res. 1993, 26, 22-29. DOI: 10.1021/ar00025a004
4. "Metal-Catalyzed Dehydropolymerization of Secondary Stannanes to High Molecular Weight Polystannanes." T. Imori, V. Lu, H. Cai and T. D. Tilley. J. Am. Chem. Soc. 1995, 117, 9931-9940. DOI: 10.1021/ja00145a001
5. "A New Mechanism for Metal-Catalyzed Stannane Dehydrocoupling Based on α-H-Elimination in a Hafnium Hydrostannyl Complex." N. R. Neale and T. D. Tilley. J. Am. Chem. Soc. 2002, 124, 3802-3803. DOI: 10.1021/ja017495s
6. "Homogeneous Catalysis with Methane. A Strategy for the Hydromethylation of Olefins Based on the Nondegenerate Exchange of Alkyl Groups and σ-Bond Metathesis at Scandium." A. D. Sadow and T. D. Tilley. J. Am. Chem. Soc. 2003, 125, 7971-7977. DOI: 10.1021/ja021341a
7. "Control of Selectivity in the Hydromethylation of Olefins via Ligand Modification in Scandocene Catalysts." F.-G. Fontaine and T. D. Tilley. Organometallics 2005, 24, 4340-4342. DOI: 10.1021/om0505460.
8. "Catalytic Functionalization of Hydrocarbons via σ-Bond Metathesis Chemistry: Dehydrosilylation of Methane with a Scandium Catalyst." A. D. Sadow and T. D. Tilley. Angew. Chem. Int. Ed. 2003, 42, 803-805. DOI: 10.1002/anie.200390213
9. "Mechanistic Aspects of Samarium-Mediated σ-Bond Activations of Arene C-H and Arylsilane Si-C Bonds." I. Castillo and T. D. Tilley. J. Am. Chem. Soc. 2001, 123, 10526-10534. DOI: 10.1021/ja011472w
10. "Activation of Arene C-H Bonds by a Cationic Hafnium Silyl Complex Possessing an α-Agostic Si-H Interaction." A. D. Sadow and T. D. Tilley. J. Am. Chem. Soc. 2002, 124, 6814-6815. DOI: 10.1021/ja025940t
11. "Octa- and Nonamethylfluorenyl Complexes of Zr(IV): Reactive Hydride Derivatives and Reversible Hydrogen Migration between the Metal and the Fluorenyl Ligand." P. Bazinet and T. D. Tilley. Organometallics 2009, 28, 2285-2293. DOI: 10.1021/om900047x
12. "Evidence for the Existence of Group 3 Terminal Methylidene Complexes." D. S. Levine, T. D. Tilley and R. A. Andersen, Organometallics 2017, 36, 80-88. DOI: 10.1021/acs.organomet.6b00394